Optical data storage systems usually employ a rotating optical disk accessed for recording and sensing by a light beam as it scans either a spiral or concentric track on the rotating disk. Usually, a laser or monochromatic light source supplies collimated light through a lense system which focusses the collimated light onto the disk. The disk reflects light through a lense system to a beam splitter. The beam splitter directs a portion of the reflected light beam onto a focus detecting apparatus which in turn indicates the degree and direction of focus error. The focus error is responded to by a focussing adjustment circuit and focussing lense moving actuator, all such as shown in U.S. Pat. No. 4,381,557.
Those optical data storage devices for storing digital computer data, for example, require a rapid and precise focussing system. Further, the focussing system should be designed to minimize construction and maintenance costs.
Focussing has been a critical area of optical recorders for many years. One of the problems with focus detectors is path length dependency, that is the fidelity of focus indication is subject to the preciseness of the path lengths between the splitter and the detector. It is, therefore, an object of the present invention to provide a focussing system in an optical apparatus which removes path length dependencies from the focus indication.
Apparatus effecting rotated light patterns are rotary shear interferometers, such as shown by Lohmann in U.S. Pat. No. 3,345,912. This patent shows a lateral shear interferometer for examining lense systems. Such a usage does not relate to a focus control system. Interference light patterns caused by lateral shear interferometers are described by Wyant and Smith in an article entitled "Interferometer for measuring power distribution of ophthalmic lenses", Applied Optics, July 1975, Vol. 14. No. 7, pages 1607-1612. This article illustrates a typical light and dark interference pattern found with lateral shear interferometers. Malacara in Optical Shop Testing, John Wiley & Sons, New York, 1978, page 105, describes the rotation of light interference patterns. Other mechanisms for rotating patterns can also be used.
Various types of detectors have been employed in optical devices. One type known is shown by Holle et al, U.S. Pat. No. 4,195,909, in FIG. 4 which is a so-called quadrant detector. Four photodetectors are arranged in a rectangular array and coupled to analyzing electronic circuits for measuring the relative light intensities impinging upon the respective detectors. In one sense, the quadrant detector is similar to an electrical bridge detector. Such quadrant detectors have been used for detecting the focussing of an optical system where a spatial frequency filter is employed with respect to a grating system. Such a detector is path length dependent; hence, is expensive to construct and maintain.
With all of the above cited references, optical recorders have not yet had a suitable path length independent focussing detection control apparatus and method.
It is therefore an object of the invention to provide a path length independent focussing system for an optical data recorder which reliably and inexpensively can indicate and control focussing of a light beam impinging upon an optical data record in the submicrometer range.